1. Field of the Invention
The present invention relates to a non-aqueous dispersion containing fine particles, in particular a non-aqueous dispersion obtained by coating said fine particles with an oil-soluble surface active agent or making the fine particles adsorb or adhere thereon an oil-soluble surface active agent and dispersing such fine particles in a water-insoluble organic solvent. In this connection, it is to be noted that the fine particles referred to herein are particles having a mean particle diameter of 1000 .ANG. or less, especially 300 .ANG. or less.
2. Description of the Prior Art
As representative non-aqueous dispersions containing fine particles, there can be enumerated (1) electrophotographic liquid developers obtained by coating pigment particles with a resin and dispersing them in a carrier liquid, and (2) gravure inks obtained by coating pigment particles with a resin and dispersing them in an organic solvent and the like. Recently, however, the study of magnetic fluids has been promoted and many investigations have been carried out for developing new uses of magnetic fluids.
The magnetic fluid referred to herein means a liquid highly magnetic substance obtained by dispersing super fine particles of a magnetic substance having a particle diameter of about 100 .ANG. stably in a solvent, said substance being provided with both fluid and highly magnetic characteristics.
The conventional process of producing this magnetic fluid, however, involves problems to be solved such as (i) the production cost is high, (ii) the products have great differences, from lot to lot, in respect of dispersed particle diameter, dispersion stability, viscosity and magnetic characteristics, and further (iii) the dispersion stability per se is not good.
The reasons for the above (i) and (ii) seem to be attributable to the fact that there are many manufacturing steps and control of each step is complicated and difficult. In addition, as the reason for the above (iii) there can be enumerated the fact that the hereinafter-described steps (a) and (b) of the conventional process employed for dispersing highly magnetic oxide particles in the water-insoluble organic solvent are defective. In more detail, in the step (a), which comprises flushing the highly magnetic oxide particles obtained by the wet process in the water-insoluble organic solvent, there is required a proper solid-liquid separating operation before flushing in the case of preparing a dispersion in which the highly magnetic oxide particles are especially super fine ones having particle diameters of 300 .ANG. or less, and further the filtering separation or centrifugal separation employed therein is inferior in efficiency. Consequently, the step (a) requires a treating step for improving the separating efficiency which in turn deteriorates the dispersion stability. In the step (b), on the other hand, grinding is effected by means of a ball mill, attriter or the like but the grinding efficiency is extremely inferior, whereby a long period of time, such as several weeks, is required and additionally it is extremely difficult to reduce the dispersed phase to a particle diameter of 1000 .ANG. or less, especially 300 .ANG. or less. In addition to these defects, the conventional process is also defective in that it is not adapted for the mass production of the above-mentioned magnetic fluid on an industrial scale.
For the purpose of eliminating these defects, a novel process for the production of magnetic fluid has been proposed in Japanese Laid Open Patent Application Nos. 13995/1976 and 65182/1979. Although these processes are each directed to a magnetic fluid using a non-aqueous organic solvent as the dispersion medium, they involve a number of manufacturing steps. Accordingly, the problems, such as production cost, dispersion stability and the like, are not solved. Furthermore, as the dispersion stability is not necessarily superior, the dispersoid dispersed in the water-insoluble organic solvent grows to have a considerably large dimension immediately after it is prepared and after time has passed. To sum up, the above-proposed processes may be said to be disadvantageous in that they cannot produce the dispersoid in the very fine particle form as is intended by the inventors of the present invention.